Temperature control systems generally include a heat transfer unit, such as an evaporator coil or heat pump, disposed to affect the temperature of air within an enclosed space. When a certain amount of frost accumulates on such heat transfer units, the heat transfer characteristics of the heat transfer unit change dramatically, causing a significant decline in the efficiency of the temperature control system.
Consequently, temperature control systems have been developed which automatically defrost their heat transfer units after a period of frost accumulation. The numerous automatic defrosting temperature control systems that have been developed have implemented various strategies to determine when a defrost operation should be initiated. The accuracy of these strategies has a significant effect on the overall efficiency of the temperature control system because, for any given heat transfer unit, the heat transfer characteristics of the heat transfer unit alter abruptly and appreciably after a specific amount of frost has accumulated on the heat transfer unit. The specific amount of frost accumulation which will cause this abrupt decline in efficiency is typically known to the manufacturer of the heat transfer unit.
Attempts have been made to design temperature control systems which automatically defrost their heat transfer units before frost accumulates beyond this known limit in order to achieve optimum efficiency. However, optimum efficiency cannot be realized if the heat transfer unit is defrosted too frequently.
U.S. Pat. No. 4,251,988 issued to Allard et. al. on Feb. 24, 1981 discloses one such temperature control system designed to initiate defrost operations just prior to the accumulation of the critical amount of frost. The Allard temperature control system initiates a defrost operation based upon the duration of a previous defrost operation. Specifically, the time required to defrost the heat transfer unit when the heat transfer unit contains the critical limit of frost accumulation is determined. Then, during operation, the temperature control system monitors the actual time required to defrost the heat transfer unit. If the actual time to defrost the heat transfer unit is greater than the time it would have taken to defrost the unit after a frost build operation of optimal duration, then too much frost was allowed to accumulate during the previous frost build operation (i.e., the defrost operation was initiated too late). On the other hand, if the actual time to defrost the heat transfer unit is less than the time it would have taken to defrost the heat transfer unit after a frost accumulation of optimal duration, then the previous frost build operation was too short (i.e., the defrost operation was initiated too soon). Based on this information, the time allowed for the current frost build operation is adjusted to be longer than the previous frost build operation if the previous defrost operation took less time than it should have, or to be shorter than the previous frost build operation if the previous defrost operation took more time than it should have.
Because the Allard defrosting strategy hinges on the duration of the actual time it takes to defrost a heat transfer unit, it is critical to accurately ascertain when the heat transfer unit is completely defrosted. If the defrost operation terminates prematurely (i.e., before the heat transfer unit is completely defrosted), then the duration of the defrost operation will not accurately reflect the time required to actually defrost the heat transfer unit. Consequently, any adjustments to the frost build time based on the duration of the defrost operation will be inaccurate.
The Allard temperature control system employs a conventional defrost thermostat (element 11) to indicate that the heat transfer unit is free of frost, and consequently to terminate the defrost operation. The defrost thermostat is placed in the vicinity of the heat transfer unit and configured to open its contacts when a predetermined defrosting temperature is exceeded. Thus, since the duration of frost build operations is determined based upon the duration of defrost operations, and the duration of defrost operations is determined by the operation of a defrost thermostat, the efficiency of the entire Allard temperature control system hinges on the accuracy of the defrost thermostat to indicate when the heat transfer unit is completely defrosted.
Based upon the foregoing, it is clearly desirable to provide an apparatus and method for checking the accuracy of a defrost completion indicator, such as a thermostat. It is further desirable to provide an apparatus and method for determining if a previous defrost operation terminated prematurely. It is further desirable to provide an apparatus and method for initiating a subsequent defrost operation if it is determined that a previous defrost operation terminated prematurely. Finally, it is clearly desirable to provide an apparatus and method for use with a temperature control system for initiating a second defrost operation if, after a first defrost operation, the heat transfer unit is not completely defrosted.